1. Field of the Invention
The present invention relates to an optical fiber, an optical amplifier including a Raman amplifier and a rare-earth doped fiber amplifier and a light source used in the optical communication system.
2. Description of the Related Art
Recently, in a field of optical communications, high-speed communication and WDM communication technique require large transmission capacity. Particularly, in the WDM communication technique, the need for a broadband optical amplifier is increased which allows batch amplification of wideband signal light. The broadband optical amplifier which meets the demand includes a Raman amplifier and a rare earth doped fiber amplifier. The Raman amplifier is configured to utilize a stimulated Raman scattering effect so as to amplify signal light. In order to cause the stimulated Raman scattering effect of the Raman amplifier efficiently, high-power pumping light is required.
In addition, in order to amplify broad band signal light, it is general to use pumping light which is obtained by multiplexing laser light of plurality of wavelengths. This sometimes makes the multiplexed pumping light output from the optical amplifier become high-power light. To provide for a broader communication band in the future, an optical amplifier capable of batch amplification over a broad spectrum is required. In order to satisfy this need, further higher power pumping light is required. In other words, pumping light of higher power is output from the optical amplifier. Here, it is also a significant purpose to design this optical amplifier so as to have a flat gain over the spectrum of amplified signal light.
The description up to now is made about the Raman amplifier. However, the above description holds true for transmitted signal light and a rare-earth doped fiber amplifier such as an EDFA. That is, light of high power is propagated. For example, even signal light of weak power is wavelength-division-multiplexed by a multiplexer such as an AWG, the signal light may become high-power light. Further, a laser element used in combination with an EDFA may output high power.
Under high-power transmission light conditions, there are more problems caused. The first problem is that of burnout of a coating material of the optical fiber. In the optical transmission system, the optical power becomes higher and higher, while for the purpose of miniaturizing an optical amplifier and a light source, the need to implement optical components at high densities has intensified. In order to realize this, it becomes necessary to bent the optical fiber with a small bending diameter in consideration of downsizing of the optical amplifier and the light source. In short, high power light should be transmit with an optical fiber housed in compact for implementation of high densities and downsizing of an optical device.
However, when an optical fiber is bent with a small bending diameter, light transmitted is leaked from the core, through the clad, coating materials to the outside of the optical fiber. Then, as described above, as the power of propagated light becomes higher, there may occur a problem that has not occurred for the conventional light intensity.
For example, when a coating material coating on the outer surface of an optical fiber absorbs light of high power leaked from the core, the optical fiber generates heat. For this reason, a portion of small bending diameter of the optical fiber is used at high temperatures. This may result in earlier deterioration of the coating material than that used in the conventional way and easy breakage of the optical fiber.
Accordingly, one object of the present invention is to solve the above-mentioned problem and to allow propagation of high-power light in an optical fiber even if the optical fiber is bent with a small bending diameter without presenting a problem such that a coating material is deteriorated due to optical power leaked out from the core of the optical fiber via the clad and the coating material.
The second problem associated with high output of optical power regards fiber fuse.
When the optical energy density in an optical fiber of transmission path is higher than a threshold, in other words in a state of high optical energy density, and various factors are added to induce core melting such as heating, a center portion of the optical fiber is first melted locally. Then, this fusion is self-propagated toward the light source. This is what is called a fiber fuse phenomenon.
In this case, the optical fiber fusing phenomenon is known to occur in a cross sectional area which is almost equal to the core diameter (approximately 10 μm) in the case of SMF (Single Mode Fiber).
This fiber fuse phenomenon propagates approximately at the speed of 1 m/s and continues as long as light transmission from an optical amplifier or light source is shut down or optical energy density becomes lower than a certain threshold. After propagation of a melted portion, propagation traces (voids) are formed in the core of the optical fiber where light cannot be transmitted.
Fiber fuse is a phenomenon that is likely to occur when high-power light is transmitted with the optical energy density being more than a certain threshold or when the fiber is heated locally. Accordingly, fiber fuse does not necessarily occur even when an area around the core is heated locally. Besides, fiber fuse occurs only when a particular condition is met, and the possibility of occurrence of fiber fuse is extremely low.
However, once fiber fuse occurs, the fiber fuse propagates toward an optical amplifier or light source, which may break a part of the optical fiber or the whole length of the optical fiber. Further, when fiber fuse reaches an optical component, or an optical device connected to the optical fiber, the optical component or optical device may be also broken, and the optical transmission path may be also broken.
For this reason, another object of the present invention is to solve the above-mentioned conventional problem and to provide an optical signal transmitting method and controlling method which allows transmission of a high-power optical signal without allowing fiber fuse to occur or propagate in an optical fiber.